1. Field of the Invention
The present invention relates to a device for measuring the flow rate of a fluid comprising a constant flow, a pulsating flow or a pulsating flow with a backward current therein, in particular, a device suited for measuring the intake air flow rate of an internal combustion engine.
2. Discussion of Background
In FIG. 31 is shown a cross-sectional view of the structure of an induction system for an automobile engine. In this Figure, reference numeral 1 designates an intake air flow rate measuring device for measuring the flow rate of intake air, reference numeral 2 designates a surge tank, reference numeral 3 designates an arrow to indicate the intake air, reference numeral 4 designates an air cleaner, reference numeral 5 designates a cleaning filter provided in the air cleaner 4, reference numeral 6 designates a throttle valve for controlling the flow rate of the intake air 3, reference numeral 7 designates an intake air passage, reference numeral 8 designates an engine combustion chamber, reference numeral 9a designates an intake manifold for introducing the intake air into the engine combustion chamber 8, reference numeral 9b designates an exhaust manifold for taking out exhaust gas after combustion, reference numeral 11 designates a flow rate measuring duct, reference numeral 12 designates a flow rate detector, reference numeral 85 designates an air inlet, reference numeral 86 designates an intake valve, and reference numeral 87 designates an exhaust valve.
The intake air 3 which has entered through the air inlet 85 is cleaned through the filter 5 in the air cleaner 4, passes through the intake air flow rate measuring device 1 and the throttle valve 6 in the intake air passage 7, the surge tank 2 and the intake manifold 9a in this order, and is introduced into the engine combustion chamber 8, being mixed with fuel. After combustion, the intake air is released to atmosphere through the exhaust manifold 9b.
It has been known that the flow fashion of the intake air 3 passing through the intake air flow rate measuring device 1 in a series of intake/exhaust strokes depends on an operation state such as engine speed and opening degree of the throttle valve 6 so that a constant stable flow with a constant flow velocity is provided in some cases and a pulsating flow with a flow velocity thereof varied with time is provided in some cases.
In the intake/exhaust strokes of an engine, the intake valve 86 starts opening in the exhaust stroke to improve trapping efficiency. In some cases, not only a forward current from an air inlet 85 toward the combustion engine 8 but also a backward current is generated in the intake pipe 7 since the exhaust gas that remains in the cylinder enters the intake valve 86 as well as the exhaust valve 87.
A conventional flow measuring device can not measure the flow rate of such a pulsating flow, in particular, a pulsating flow with a backward current therein, and the conventional device produces a considerable error in measurement of the flow rate of such a pulsating flow. Although the error has been reduced by software for correction in such a case, the measurement of the flow rate has a limited measuring accuracy, and the correction contributes to an increase in cost. From this viewpoint, it is extremely desirable that the intake air flow rate measuring device 1 basically has a function to detect a backward current, in terms of improvement in a measuring accuracy and a reduction in cost.
Now, explanation of the intake air flow rate measuring device 1 will be made. In order that the measurement of the intake air flow rate in an internal combustion engine becomes decreasingly less susceptible to drift or turbulence caused by a bent portion of the intake air passage 7 or the air cleaner 4, the flow rate measuring duct, which is a size smaller than the intake air passage, has been provided in the intake air passage so as to have a longitudinal axis thereof extended substantially parallel to the flow of a fluid to be detected, and the flow rate detector 12, such as a flow velocity sensor, has been in turn provided in the flow rate measuring duct to rectify the flow near to the detector, producing a stable output.
This arrangement has created a problem in that the flow rate detector 12 in the flow rate measuring duct 11 can not stably detect a flow rate of a fluid to be detected since the provision of the flow rate measuring duct 11 produces unstable vortexes or separation of the flow near to an inner wall of the flow rate measuring duct 11 to disturb the flow passing through the flow rate measuring duct 11. If the flow separates at an inlet of the flow rate measuring duct 11, the separation region has a thickness thereof increased toward a downstream direction. It is known that gas is irregularly disturbed by a shear force in the vicinity of the boundary between the separation region and a principal current portion since the separation region and the principal current portion have different flow velocities. The irregular disturbance has contributed to generation of an error in flow rate measurement.
In order to solve this problem, it has been proposed in JP-A-604813 that the flow rate measuring duct with the rectifying function stated above has small holes to reduce separation currents and vortexes caused at the inlet of the flow rate measuring duct so as to equalize the flow velocity distribution in the flow rate measuring duct. The details of this arrangement will be explained, referring to FIGS. 32(a) and 32(b). FIG. 32(a) is a cross-sectional side view, and FIG. 32(b) is a front view. In these Figures, reference numeral 100 designates an intake pipe, reference numeral 101 designates the flow rate measuring duct, reference numeral 102 designates an elastic heater element for measuring a flow rate, reference numeral 103 and 104 designate temperature-dependent elements, reference numeral 105 designates a first supporter, reference numeral 106 designates a second supporter, reference numeral 107 designates small holes, and reference numeral 108 designates a stay.
When the resistance wire 102 is energized and heated, and when air flows across the resistance wire in a forward direction, the temperature-dependent resistance wire 103 is cooled by the air flow supplied from an upstream direction. Since the air that has been heated by an upstream portion of the temperature-dependent resistance wire 103 passes across temperature-dependent resistance wire 104 at that time, a temperature difference due to heating of the intake air is provided between the temperature-dependent resistance wire 103 and the temperature-dependent resistance wire 104. The temperature difference varies, depending on the caloric value of the resistance wire 102 and the mass flow rate of the intake air. The static pressure on an inner wall of the flow rate measuring duct 101 becomes smaller than the static pressure outside the flow rate measuring duct since the flow velocity in the flow rate measuring duct 101 is slower than that outside the flow rate measuring duct 101 because of the presence of friction loss against the inner wall in the flow rate measuring duct 101. The difference in both static pressures creates currents which are directed into the flow rate measuring duct 101 from outside the flow rate measuring duct 101 through the small holes 107. Since the gas that has flowed into the flow rate measuring duct 101 through the small holes 107 enters the separation region to reduce the velocity difference between the principal current portion and the separation region, a velocity boundary layer comes closer to the inner wall of the flow rate measuring duct 101, decreasing the disturbance in the flow velocity. The publication states that this arrangement can transfer the heat from the heater 102 to the temperature-dependent element 104 in stable fashion to improve the flow rate measuring accuracy.
However, this proposal does not take into account a problem in that, when the flow rate measuring duct 101 is provided with a pulsating flow with a flow velocity thereof varied with time, the flow velocity in the flow rate measuring duct 101 is lowered under the influence of vortexes caused in a rear flow behind the flow rate measuring duct, producing an error in flow rate detection.
An object in a pulsating flow has totally different flow fashion from an object in a constant flow. An object in an accelerating flow has quite different flow fashion from an object in a decelerating flow. In particular, when the conventional flow rate measuring duct 11 is provided in a pulsating flow, a significant error in flow rate detection is produced in deceleration, which will be explained.
Before explaining the problem that is caused by the provision of the flow rate measuring duct 11 in a pulsating flow, an explanation of a case in which a flat plate is provided in a constant flow or a pulsating flow so as to extend along the flow will be made.
In FIG. 33 is shown shear currents that merge through a flat plate 21 put in a constant flow so as to extend in parallel with the flow, wherein an unstable shear layer is produced at the boundary surface between the shear currents, two-dimensional cyclic vortexes 98 are produced, the cyclic vortexes change into discrete vortexes 55 and the discrete vortexes eventually collapse. It is known that the currents mix together in a region having a certain expansion angle 99. As shown in FIG. 34, the mean flow velocity distribution in that time becomes flatter in the mixing region as the flows move downstream. As a result, the shear is gradually eased.
In the case of an accelerating pulsating flow, the expansion angle 99 of the mixing region becomes smaller than that in the case of a constant flow because of addition of a potential flow to the flow just prior to acceleration as shown in FIG. 35.
In the case of a decelerating pulsating flow, the expansion angle 99 of the mixing region becomes larger and the discrete vortexes 55 become more massive than those in a constant flow as shown in FIG. 36.
Since the flow rate measuring duct 11 is provided by forming the flat plate 21 into a cylindrical shape, the flow fashion in the rear flow behind the flat plate 21 expands in a circumferential direction of the flow rate measuring duct 11 with respect to a longitudinal axis 18 thereof. In the case of a constant flow, annular-shaped vortexes 90 are produced from a downstream end of the flow rate measuring duct 11 because of the presence of shear force caused by a velocity difference between air currents 23 and 24 in and outside the flow rate measuring duct 11 as shown in FIG. 37. The annular-shaped vortexes 90 diffuse, being changed into the discrete vortexes 55 by the mixing region having a certain expansion angle 99. In this Figure, reference numeral 52 designates a distance required for the vortexes to collapse. A shorter distance indicates faster development in collapse.
In the case of an accelerating pulsating flow, the current 23 in the flow rate measuring duct 11 is almost the same as the current 24 outside the flow rate measuring duct 11 as shown in FIG. 38 since the mixing region is constricted.
In the case of a decelerating pulsating flow, the flow velocity in the flow rate measuring duct 11 is significantly lowered in comparison with the case of a constant flow as shown in FIG. 39 since the discrete vortexes 55 become massive and occupy a wide portion near to the outlet of the flow rate measuring duct 11 so as to prevent the current 23 from going out from the flow rate measuring duct. As a result, the flow rate of the current 24 outside the flow rate measuring duct 11 is increased by a decrease in the flow rate of the current 23 in the flow rate measuring duct 11, changing a separation ratio of both currents in and outside the flow rate measuring duct 11. The separation ratio means a ratio of the flow rate in and outside the flow rate measuring duct 11. If the flow rate measuring device 1 is set so that the relationship between a total flow rate and an output from the flow rate detector 12 are checked with respect to a constant flow, and if the flow changes from a constant flow into a pulsating flow, the flow velocity in the flow rate measuring duct 11 is significantly lowered and prevents the flow rate detector 12 from correctly detecting a flow rate, causing a problem in that the flow rate measuring device 1 produces an error in flow rate detection.
If a backward current is produced, the discrete vortexes 55 which have become massive during deceleration are carried on the backward current to move upstream, being separated into portions in and outside the flow rate measuring duct 11 as shown in FIG. 40. The vortexes that have come into the flow rate measuring duct 11 collide against the flow rate detector 12 to disturb the current near to the flow rate detector. This creates a problem in that an error in flow rate detection is produced since the flow rate detector 12 detects a variation in the flow velocity disturbed by the discrete vortexes 55 irrespective of the principal current.
Although the provision of the small holes in an upstream portion of the flow rate measuring duct 101 shown in FIG. 32 provides a rectifying effect in the flow rate measuring duct 101 to some degree in the case of a constant flow, this arrangement does not at all take into account the problem in that, in the case of a pulsating flow, the separation ratio of the currents in and outside the flow rate measuring duct 101 is varied under the influence of the vortexes generated in the rear flow to produce an error in flow rate detection.